linux-kernel - Re: [PATCH RFC 0/5] mm: process_vm_mmap() -- syscall for duplication a process mapping

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Date:   Thu, 16 May 2019 15:32:06 +0200
From:   Jann Horn <jannh@...gle.com>
To:     Kirill Tkhai <ktkhai@...tuozzo.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Michal Hocko <mhocko@...e.com>, keith.busch@...el.com,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        pasha.tatashin@...cle.com,
        Alexander Duyck <alexander.h.duyck@...ux.intel.com>,
        ira.weiny@...el.com, Andrey Konovalov <andreyknvl@...gle.com>,
        arunks@...eaurora.org, Vlastimil Babka <vbabka@...e.cz>,
        Christoph Lameter <cl@...ux.com>,
        Rik van Riel <riel@...riel.com>,
        Kees Cook <keescook@...omium.org>, hannes@...xchg.org,
        npiggin@...il.com,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Roman Gushchin <guro@...com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Hugh Dickins <hughd@...gle.com>,
        Jerome Glisse <jglisse@...hat.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        daniel.m.jordan@...cle.com,
        kernel list <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        Linux API <linux-api@...r.kernel.org>
Subject: Re: [PATCH RFC 0/5] mm: process_vm_mmap() -- syscall for duplication
 a process mapping

On Wed, May 15, 2019 at 5:11 PM Kirill Tkhai <ktkhai@...tuozzo.com> wrote:
> This patchset adds a new syscall, which makes possible
> to clone a mapping from a process to another process.
> The syscall supplements the functionality provided
> by process_vm_writev() and process_vm_readv() syscalls,
> and it may be useful in many situation.
[...]
> The proposed syscall aims to introduce an interface, which
> supplements currently existing process_vm_writev() and
> process_vm_readv(), and allows to solve the problem with
> anonymous memory transfer. The above example may be rewritten as:
>
>         void *buf;
>
>         buf = mmap(NULL, n * PAGE_SIZE, PROT_READ|PROT_WRITE,
>                    MAP_PRIVATE|MAP_ANONYMOUS, ...);
>         recv(sock, buf, n * PAGE_SIZE, 0);
>
>         /* Sign of @pid is direction: "from @pid task to current" or vice versa. */
>         process_vm_mmap(-pid, buf, n * PAGE_SIZE, remote_addr, PVMMAP_FIXED);
>         munmap(buf, n * PAGE_SIZE);

In this specific example, an alternative would be to splice() from the
socket into /proc/$pid/mem, or something like that, right?
proc_mem_operations has no ->splice_read() at the moment, and it'd
need that to be more efficient, but that could be built without
creating new UAPI, right?

But I guess maybe your workload is not that simple? What do you
actually do with the received data between receiving it and shoving it
over into the other process?